Lots of things could be going on to cause your period to be lasting for three weeks. Depending on how thick your uterine lining is, sometimes it just does a really bad job of falling off quickly and flowing out through your vagina, so you have a small amount of bleeding daily for a few weeks. If you started off with a more “normal” period for you but there has been just some spotting every day since to make up the three weeks, this may not be anything concerning. If it happens cycle after cycle, it’s a hassle and may indicate a minor hormone imbalance and you should be seen by your healthcare provider. If, each day for the three weeks your flow has been pretty steady and reddish, (as opposed to spotting which is usually brown), it’s time to go to your healthcare provider now. Of course pregnancy can cause hormones to be out of whack and, believe it or not, some women have small amounts of bleeding for a while every day at the beginning of pregnancy so if there’s any chance you are pregnant, I would encourage you to do a pregnancy test or go to your healthcare provider to find out.
A period that lasts longer than seven days is considered prolonged. It's not unusual for girls to have irregular, brief, or slightly prolonged periods. But three weeks may be pushing it. There are many possible causes for prolonged bleeding but, not being a doctor, I wouldn't even like to venture a guess. What I would like is for you to make an appointment with your healthcare provider. I'm sure you're feeling concerned and maybe even a little scared, but he or she will help you figure out what's going on. And be sure to read Dr. Molly's answer — she'll have some medically-based words of wisdom for you.
Goodness girl! As if a week weren’t enough? While my periods have never been like clockwork, I’ve never had one last for three weeks. You really should talk to your healthcare provider about any abnormal or extensive bleeding. Of course it’s not always the easiest thing to bring up in normal conversation, but keep mind every woman goes through it (you’re not alone!) and there is no shame in asking a question that concerns your health!